1. Field of the Invention
The present invention relates to a black synthetic quartz glass with a transparent layer and a method for producing the same. The black synthetic quartz glass with a transparent layer of the present invention is suitably used for an optical component such as a spectroscopic cell, an infrared heat-absorbing member, a plasma etching-resistant member, a light-shielding member for a semiconductor production apparatus, and the like.
2. Description of the Related Art
A quartz glass is used in the field of optics such as a spectroscopic cell through utilization of its satisfactory light transmission property over ultraviolet to infrared regions and low thermal expansion property. Conventionally, in this field, a black glass obtained by adding a trace amount of a transition metal oxide to the quartz glass has been used for a part where local light shielding is required, and an optical cell has been produced by, for example, bonding the black glass to a transparent quartz glass through thermocompression bonding or the like. In recent years, however, in association with progressive reductions in size and thickness of the optical cell, there have been some cases where the conventional black glass has been insufficient in terms of light-shielding property. Accordingly, there has been a demand for a black quartz glass which has higher light-shielding property, is free of bubbles, foreign matter, and the like, thereby being highly homogeneous, and can be readily bonded to the transparent quartz glass.
Further, the quartz glass also has features of heat resistance and high chemical purity, and hence has been widely used for, for example, jigs for semiconductor production. In this regard, in recent years, a heat treatment step in a semiconductor production process has been more and more frequently performed in a high temperature region of more than 1,000° C. Accordingly, there has been an increasing demand for high heat resistance. Further, in a rapid heating process using infrared light, heat loss due to the transparent quartz glass through which the infrared light passes has been a problem, and hence there has also been a demand for a member for shielding components except an object to be heated from infrared irradiation. For these reasons, there has been a demand for development of a black quartz glass which is excellent in heat resistance, effectively shields infrared rays, has rapid heating and cooling properties, is excellent in thermal insulation property, is resistant to thermal shock at the time of rapid heating and cooling, and moreover, is free of metal impurities which cause process contamination.
As a black glass containing silica as a main component, the following glasses have been conventionally known. For example, Patent Document 1 proposes a black quartz glass obtained by adding a metal compound to a quartz glass. However, the black quartz glass of this kind is not sufficient in light-shielding property in some cases, and the metal component contained therein may cause process contamination. Accordingly, application of such glass to the semiconductor production field involves difficulties.
Further, Patent Document 2 proposes a black quartz glass obtained by adding an organic binder capable of serving as a carbon source to silica powder, subjecting the mixture to heat treatment to generate decomposed carbon, and then firing the resultant to dissolve the carbon in a glass network. However, such glass body having carbon dissolved therein is known to have mechanical and thermophysical properties that are different from those of a general quartz glass, such as increased hardness and increased high-temperature viscous property. Its thermal expansion rate is considered to change as well. Accordingly, such glass body has a difficulty such as causing a crack when used by being bonded or fitted to a general transparent quartz glass. In addition, in a heating atmosphere, the following problems arise. Carbon in the vicinity of a surface scatters into an ambient atmosphere to exert an adverse influence, and silica and carbon undergo reactions to generate air bubbles, which makes it impossible to perform welding or flame processing. In addition, for example, air bubbles are generated from an interface between the transparent quartz glass and the black quartz glass during use under heating, causing a crack in a product being used.
Further, Patent Document 3 describes a method involving placing a hydroxy group-containing porous quartz glass body having a desired shape in an atmosphere of a volatile organosilicon compound, heating the glass body to cause a reaction, and then heating and sintering the resultant in a vacuum, thereby forming a quartz glass transparent layer having a thickness of about 1 mm to 10 mm on a surface of the black quartz glass body having a desired shape. In this method, however, it is difficult to produce quartz glass jigs having various shapes, and it is also difficult to appropriately adjusting the thickness of the transparent layer while keeping a degree of blackness that satisfies required light-shielding property and emissivity in the infrared region.